When Joey Garza opened a sports-therapy studio in his Heights home in late 2010, he accepted only cash. He wanted to take credit cards, too, but setting up a system to do so would have been "crazy expensive," he said.

Then he discovered the mobile system called Square, which allows him to accept credit card purchases on his iPad or iPhone. The small plug-in device is basically free, and Garza pays a flat rate for each transaction with no monthly fees. It's made a big difference.

"People don't pay with cash anymore," he said.

Garza's studio, Urban Therapy, has grown enough to allow him to open a storefront in the Heights, and he now also offers fitness classes, yoga and massage.

He credits Square in part for his success.

Square, which entered the market in 2010 and currently processes more than $6 billion in payments annually, has worked for many other small-business owners who felt priced out of traditional point-of-sale processing systems.

"Square has been really great at the user experience side of things: taking an existing behavior - swiping your card - to create a new system of payment," Maurer said. "You just plug the Square device into an earphone jack and - presto!"

Twitter co-founder

The San Francisco-based company, founded by Twitter co-founder Jack Dorsey, competes with such mobile providers as Intuit, PayPal, Level­Up and Google Wallet. Maurer said Verifone is Square's top competitor even though it's not a mobile device, because "that's the main entity whose piece of the pie they're trying to get at."

Other mobile-payment systems may require consumers to learn something new, but Square technology is "so simple, you don't even notice it," Maurer said. He said that works in the company's favor since consumers can be nervous around payment technology because of privacy, security and accuracy issues.

More than half the small-business owners using Square had previously not accepted credit cards, company spokeswoman Lindsay Wiese said. In the first six months of this year, she added, Square doubled its number of users to about 2 million.

Users, like Garza, pay a flat 2.75 percent of each transaction to Square. While traditional point-of-sale systems may charge fees as low as 1.7 percent, many of those plans require a monthly fee and substantial sign-up and other fees, he said.

"Square is perfect for us," said Amalia Pferd, co-owner of Good Dog Hot Dogs, a local food truck.

Good Dog customers initially were taken aback by Square - "Hey, wow, how cool a small little food truck can have this kind of technology" - but they have been increasingly less surprised, said her business partner Daniel Caballero, who believes it makes his operation seem more professional.

Cash no longer king

People are more likely to spend more with credit cards, Caballero said, because they may not have enough cash on hand to buy every food item they want.

A few days ago, Square announced a pricing option for small businesses: A $275 monthly fee with no processing charges for each transaction.

Square also works with Android devices.

The service offers an app, called Pay With Square, that allows consumers to pay for items using their own phones once they've registered with a participating retailer.

When they visit the store, their name and photo appear on the merchant's register, and the customer pays without having to present a credit card.

Earlier this month Starbucks announced that its customers will be able to use Pay with Square.

Wiese said the partnership is a big move for the company.

"By working with Starbucks, Square can significantly advance our scale and accelerate our ability to provide merchants with the tools they need to grow their business," she said.

Retailing revolution

Maurer said he doesn't believe Square will displace traditional point-of-sale machines at many major retailers because larger chains are still going to want to accept other forms of payment, such as PIN-debit transactions and loyalty cards, and the cost of replacing existing terminals may be too expensive, he said.

It is a revolutionary time in the point-of-sale payment world, Maurer said.

For example, retailers that use traditional point-of-sale machines are trying to come up with their own mobile systems. He said Wal-Mart, Best Buy and Target have joined forces to create Merchant Customer Exchange, a mobile payment network.

Apple Store locations have offered customers such an option since 2006, Maurer said.